The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Diesel engines emit exhaust gas that includes carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). A diesel exhaust treatment system reduces the levels of CO, HC, and NOx in the exhaust gas. The diesel exhaust treatment system may include a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), and a selective catalytic reduction (SCR) system. The DOC oxidizes CO and HC to form carbon dioxide and water. The DPF removes diesel particulate matter from the exhaust gas. The SCR system reduces NOx.
The SCR system involves injecting a reducing agent (e.g., urea) into the exhaust gas upstream from an SCR catalyst. The reducing agent forms ammonia that reacts with NOx in the SCR catalyst. The reaction of ammonia and NOx in the SCR catalyst reduces the NOx and results in the emission of diatomic nitrogen and water. When excess reducing agent is injected into the exhaust gas, the excess reducing agent may form excess ammonia that passes through the SCR catalyst without reacting. An additional catalyst may be used downstream of the SCR catalyst to remove the excess ammonia.